If you live in the U.S.,
Digi-Key has 700 in stock.
SparkFun usually has them and they have a
circuit board with two chips (for stereo) and a 3.5mm pass-through connector.
The Adafruit microphone board should work, depending on the sound level, etc. You do need a board like that, or you'll need a preamp because you only get a few millivolts out of a microphone. That particular board has automatic gain control which can he helpful for a display/effect but it's no good if you want to make "measurements".
That board (like most microphone boards) has a biased-output so it can be connected directly to an Arduino. (The Arduino input has to be biased because Arduino can't handle the negative half of an AC audio signal). But the input to the MSGEQ7 should not be biased so make sure there is a capacitor in series between the microphone-board output and the MSGEQ7 input.
...If this is for music, it's best to make a direct line-level (or headphone-level) connection because the mic will pick-up room noise.